---
stage: Verify
group: Pipeline Execution
info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/product/ux/technical-writing/#assignments
type: concepts, howto
---

# Make Docker-in-Docker builds faster with Docker layer caching

When using Docker-in-Docker, Docker downloads all layers of your image every
time you create a build. Recent versions of Docker (Docker 1.13 and later) can
use a pre-existing image as a cache during the `docker build` step. This significantly
accelerates the build process.

## How Docker caching works

When running `docker build`, each command in `Dockerfile` creates a layer.
These layers are retained as a cache and can be reused if there have been no changes. Change in one layer causes the recreation of all subsequent layers.

To specify a tagged image to be used as a cache source for the `docker build`
command, use the `--cache-from` argument. Multiple images can be specified
as a cache source by using multiple `--cache-from` arguments. Any image that's used
with the `--cache-from` argument must be pulled
(using `docker pull`) before it can be used as a cache source.

## Docker caching example

This example `.gitlab-ci.yml` file shows how to use Docker caching:

```yaml
default:
  image: docker:20.10.16
  services:
    - docker:20.10.16-dind
  before_script:
    - docker login -u $CI_REGISTRY_USER -p $CI_REGISTRY_PASSWORD $CI_REGISTRY

variables:
  # Use TLS https://docs.gitlab.com/ee/ci/docker/using_docker_build.html#tls-enabled
  DOCKER_HOST: tcp://docker:2376
  DOCKER_TLS_CERTDIR: "/certs"

build:
  stage: build
  script:
    - docker pull $CI_REGISTRY_IMAGE:latest || true
    - docker build --cache-from $CI_REGISTRY_IMAGE:latest --tag $CI_REGISTRY_IMAGE:$CI_COMMIT_SHA --tag $CI_REGISTRY_IMAGE:latest .
    - docker push $CI_REGISTRY_IMAGE:$CI_COMMIT_SHA
    - docker push $CI_REGISTRY_IMAGE:latest
```

In the `script` section for the `build` job:

1. The first command tries to pull the image from the registry so that it can be
   used as a cache for the `docker build` command.
1. The second command builds a Docker image by using the pulled image as a
   cache (see the `--cache-from $CI_REGISTRY_IMAGE:latest` argument) if
   available, and tags it.
1. The last two commands push the tagged Docker images to the container registry
   so that they can also be used as cache for subsequent builds.
